ity. Professor J. J. Nassau, of the department of mathematics and astronomy of Case School of Applied Science, and Dr. G. Strömberg, of the staff of the Mount Wilson Observatory, have given very great assistance in the mathematical analysis, and have developed solutions of various parts of the problem, and also a complete least-squares solution of the general problem. A definitive numerical calculation will require several months of continuous work and is now in progress.

The ether-drift experiments at Mount Wilson during the last four years, 1921 to 1925, lead to the conclusion that there is a positive displacement of the interference fringes, such as would be produced by a relative motion of the earth and the ether at this observatory, of approximately ten kilometers per second, being about one third of the orbital velocity of the earth. By comparison with the earlier Cleveland observations, this suggests a partial drag of the ether by the earth, which decreases with altitude. It is believed that a reconsideration of the Cleveland observations, from this point of view, will show that they are in accordance with this presumption, and will lead to the conclusion that the Michelson-Morley experiment does not and probably never has given a true zero result. A complete calculation of experiments, to be made in the immediate future, should give definite indications regarding the absolute motion of the solar system in space.